When sound is reproduced, for example, a waveform signal is converted into an electrical signal, and a drive system is driven by the electrical signal, to emit the sound.
The conversion process for performing acoustic radiation from an electronic circuit through an oscillation system is called electroacoustic transduction, and elements and devices that perform electroacoustic transduction are called electroacoustic transducers. For example, speakers, headphones, and the like are electroacoustic transducers.
Speakers that are widely used at the present time are electrodynamic or dynamic speakers. Speakers of other types include piezoelectric speakers, electrostatic speakers, and the like.
An electroacoustic transducer of an electrodynamic type takes advantage of the fact that, when an object electrified in a vertical direction is placed in a uniform magnetic field, force is generated in the object in a direction perpendicular to the magnetic field and the current.
An acoustic signal has an alternating current and a variable current direction. Therefore, the direction of the force changes, and the object (an electrical conductor) vibrates with the change. Such an electroacoustic transducer is called a dynamic speaker.
Meanwhile, a piezoelectric electroacoustic transducer takes advantage of the fact that, when voltage is applied from an external power supply to a structure formed by bonding a metal diaphragm to a piezoelectric crystal plate, electrostatic force acts on the positive and negative ions that constitutes the piezoelectric crystal plate, and stress is generated to distort the crystal.
For example, the piezoelectric crystal plate is subjected to stress that acts to widen the area. However, one side of the piezoelectric crystal plate is constrained by the metal diaphragm, and therefore, the piezoelectric crystal plate expands, to cause warpage in the metal diaphragm. Further, if the voltage direction is reversed, the piezoelectric crystal plate is subjected to stress that acts to narrow the area. However, one side of the piezoelectric crystal plate is constrained by the metal diaphragm. Therefore, the piezoelectric crystal plate shrinks, and the metal diaphragm warps toward the opposite side. Such an electroacoustic transducer is called a piezoelectric speaker.
Further, an electroacoustic transducer of an electrostatic type takes advantage of the fact that an electrostatic field is formed when a DC voltage is applied between an electrically conductive diaphragm and a fixed plate that are brought close to each other. If an alternating current that is an acoustic signal is applied in this situation, the attractive force of the electrostatic field changes due to the voltage change, and the diaphragm moves. Such an electroacoustic transducer is called an electrostatic (capacitor) speaker.
Meanwhile, resistance indicates the degree of difficulty for electricity to flow. If the resistance is high, electric current hardly flows even when the same voltage is applied. The resistance often depends on the frequency of an AC voltage, and the resistance at each frequency is called impedance.
In the meantime, a dynamic speaker requires a magnetic circuit system such as a magnet as a component. Therefore, the unit thickness and the unit weight of the dynamic speaker exceed certain levels.
For example, in a case where the speaker is carried around as a portable device, or where the speaker is attached to a wall surface and is then used, thinness and lightness are critical for the speaker to be commercialized. Further, in a case where the speaker is used as an in-vehicle device in a private vehicle or the like, thinness and lightness of the speaker are also critical in commercialization.
Here, a piezoelectric speaker and an electrostatic speaker have the advantage that both speakers are lighter than a dynamic speaker, requiring no magnetic circuit systems. However, there is a tendency that it is difficult for a piezoelectric speaker and an electrostatic speaker to secure sound pressure in low frequency ranges.
To counter this, a method of increasing the speaker area has been suggested as a method for increasing sound pressure in low frequency ranges. However, if the speaker area is increased, manufacturing becomes difficult due to the increase in size, and further, freedom in installation is lost.
On the other hand, a piezoelectric speaker and an electrostatic speaker have capacitive impedance. In other words, a piezoelectric speaker and an electrostatic speaker have such characteristics that the impedance is high in low frequency ranges and is low in high frequency ranges, unlike the characteristics of a dynamic speaker whose impedance stays within the range of 4 to 16 ohms in all frequency ranges.
For example, in a case where a plurality of speakers are driven, the impedance becomes even lower if the speakers are connected in parallel, and, in a dynamic speaker, the impedance in the audio band might become too low. In such a case, if the impedance of the speakers is lower than the output impedance of the amplifier, the current output to be supplied to the speakers is restricted, and the performance of the speakers and the amplifier is degraded.
Meanwhile, in the trend of a wider variety of uses of speakers, there is a method of increasing the effects of advertising by giving directivity to the sound to be output outdoors for digital signage.
In addition, as for personal use, directivity is given so that persons do not bother each other in a case where a plurality of persons in the same house listen to different sounds at the same time, directivity is given in a navigation system that is to give warnings only to the driver in a vehicle, or the like, for example.
In such cases, if the volume of sound is simply increased, the sound becomes nothing more than noise for the others, and it cannot be said that a preferred audio environment is successfully provided.
In view of this, techniques for realizing sound reproduction having directivity have been suggested.
For example, a parametric speaker specialized in directivity has been suggested as such a technique (see Patent Document 1, for example). With this technique, a plurality of ultrasonic transducers are provided to improve the directivity characteristics of ultrasonic waves. In addition, as for the material of ultrasonic transducer speakers, a piezoelectric material or an electrostatic material is used, and a plurality of ultrasonic transducer speakers are connected in parallel.
Further, a speaker system has also been suggested as a technique for realizing sound reproduction having directivity. In the speaker system, a plurality of dynamic speakers are used, and the dynamic speakers are disposed linearly and densely, to improve directivity characteristics (see Patent Document 2, for example).